sync data in to Dynamics 365 Sales to Legacy web apps vice versa

There's no easy answer to this one - there are myriad ways you can achieve a near real-time integration. Dataverse has the web API and the organisation service, there are Power Platform connectors; the recommendation for the way you connect and the technology you utilize will be based on factors such as data volumes, business criticality, licensing/cost requirements... I recommend you make a post here https://community.dynamics.com/365/sales/f/dynamics-365-for-sales-forum with some further details about the requirements of your integration and the function of your legacy system.